A coffee habit, coupled with regular exercise, may help prevent skin cancers better than either factor alone, new research suggests.

The study was done only with animals, however, and it's not a reason to abandon standard sun-protection habits.

"You should not give up the sunblock," said Dr. Allan H. Conney, senior author of the study, published in this week's Proceedings of the National Academy of Sciences.

The findings aren't entirely new. "In earlier studies, we found caffeine and exercise -- either one by themselves -- inhibited ultraviolet light-induced skin cancer in mice," said Conney, the director of the Laboratory for Cancer Research at the School of Pharmacy at Rutgers, the State University of New Jersey.

But the new research shows that "the combination [of the two] works better," he said, providing a dramatically better anti-cancer result.

Both caffeine and exercise seem to help kill the UVB-damaged cells before malignancy sets in. "We really don't know how that happens," Conney said.

In the study, his team looked at four groups of hairless mice. The rodents' exposed skin is very vulnerable to the sun.

One group was given caffeinated water to drink each day, the equivalent of a person drinking a couple of cups of coffee a day, Conney said. Another group ran voluntarily on a running wheel, the equivalent of a person running two or two and a half miles every day, he said. (These mice will happily go on an exercise wheel if one is available, Conney said.) A third group had both the caffeine and the exercise, while a fourth group had neither and served as the control group.

The mice in all four groups were exposed to lamps that generated UVB radiation that damaged the skin cells' DNA.

While some degree of healthy, programmed skin cell death ("apoptosis") was seen in all four groups of mice, the caffeine drinkers and exercisers were best at killing off the damaged cells, the researchers found.

To find out how different the four groups were in terms of killing off damaged skin cells, the researchers looked at physical changes in those cells. They also looked at chemical markers, such as enzymes, involved in killing damaged cells.

The differences were dramatic. The caffeine drinkers showed a 96 percent increase in damaged cell death compared to the control group and the exercisers showed a 120 percent increase. Even more significant, the mice that drank caffeine and ran on the training wheel had a nearly 400 percent increase in cell death of damaged cells.

Whether this combination would work in people is not known, Conney said, although some research has found that caffeine and exercise does reduce certain cancer risks. He said he would like to do a clinical study in humans next.

More than a million non-melanoma skin cancers are diagnosed in the United States annually, according to the American Cancer Society. About 62,190 cases of melanoma, the most deadly skin cancer type, will be diagnosed this year.

A spokesman for the Skin Cancer Foundation urged caution in interpreting the study findings, however. "It will take years of extensive testing to determine whether this will be a worthwhile concept before you can say anything specific about it," said Dr. Michael Gold, founder of the Gold Skin Care Center in Nashville, Tenn.

"Mice and humans are very different. That said, we do know that caffeine applied topically has been popular as a 'cosmeceutical' anti-aging ingredient and might be useful in helping prevent non-melanoma skin cancers," Gold said. "The concept of systemic caffeine needs to be addressed further. We also know that moderate exercise is an immune moderator and can help ward off cancers and other diseases."

He echoed standard advice to wear sunscreen when out in the sun. "If you are exercising outside you must wear sunscreen no matter what," Gold said. "If you don't protect yourself from the sun while exercising outdoors you are increasing your risk of getting non-melanoma skin cancers and melanoma. Protecting yourself from the sun is currently the only proven way to prevent skin cancer."